This invention refers to a method for producing nuclear fuel pellets of the MOX (=mixed oxide) type, comprising the steps of
preparing an U-Pu oxide blend powder having a Pu content in excess of the finally desired value,
preparing an uranium oxide powder,
mixing adequate quantities of both powders in order to achieve the desired plutonium content,
compacting and sintering the mixture for obtaining said pellets.
Such a method is known under the term MIMAS (xe2x80x9cMIcronized MASter Blendxe2x80x9dxe2x80x94see for example D. Haas, M. Lippens xe2x80x9cMOX FUEL FABRICATION AND IN-REACTOR PERFORMANCExe2x80x9d, Proc. of the Internat. Conference on Future Nuclear Systems, GLOBAL 97, p.489 à 494). This separate preparation of a powder free of plutonium reduces the volume of plutonium containing powder that has to be milled, and allows the production of fuel pellets of various plutonium contents with a unique plutonium treatment chain by changing only the rate of admixed uranium powder.
The commercial powders currently used, however, result in a final product which is heterogeneous, i.e. contains large particles rich in plutonium oxide dispersed within an uranium oxide matrix whose grain size is below 10 xcexcm. This heterogeneity leads to two major drawbacks:
During irradiation localised higher fissile material concentrations lead to high local burnups, to fission damages and to gas release. To limit this gas release large UO2 grains are recommended, provided that they are produced without additives that might lead to detrimental fuel behaviour during irradiation and might also lead to difficulties during reprocessing.
During reprocessing the dissolution of the burned-up fuel in nitric acid is hindered by regions rich in plutonium, which is notoriously insoluble.
The present invention aims to overcome these drawbacks and to propose a method as indicated above which leads to fuel pellets of the MOX type in which the distribution of plutonium throughout the pellet is substantially more homogeneous.
This aim is achieved by the method as defined in claim 1. As far as preferred embodiments of this method are concerned, reference is made to the secondary claims.